ARTERIOVENOUS MALFORMATION - A hemangioma, or arteriovenous malformation is a tangle of abnormal vessels that forms an abnormal communication between the arterial and venous systems. Most are developmental (congenital). If large enough, they may produce a shunt of sufficient magnitude to raise the cardiac output. AVM's may occur in the brain, brainstem, and spinal cord, where they may cause headaches, seizures, or bleeding (subarachnoid hemorrhage).